Reproduction in macropodid marsupials is distinguished by a short gestation which can be greatly extended by a period of embryonic diapause or delayed implantation. These mammals offer a unique opportunity to study the control of embryonic development. Their placental attachment is a simple inter-digitation of maternal and fetal tissue, but the yolk-sac placenta is apparently an endocrine organ. The resumption of embryonic growth after delay is a complex interaction of the blastocyst and uterus. Steroid hormones have been implicated in the initiation of secretory activity by the uterus, and blastocyst development, while the neural pathway from teat to the hypothalamus maintains the prolactin production inhibiting implantation. Information has now been collected on the development of the embryo, on the steroid metabolism of the embryo and uterus, and preliminary data on the metabolic activity of the embryo after the resumption of blastocyst growth and the formation of the yolk sac placenta. The main objective of the research proposed here is to clarify the timing of these events, and to extend our studies on the nature of the interactions between blastocyst, uterus and mammary gland. We propose to: (1) measure incorporation of glucose and uridine in vitro in blastocysts collected daily before entry into diapause and for 15 days following resumption of development, and to measure metabolism of labelled steroid precursors during the same times by the corpus luteum, endometrium and embryonic tissue (2) describe the anatomy of the vascular supply to the uterus, and measure steroids in the plasma at various sites close to the corpus luteum and uterus (3) investigate the corpus luteum inhibition of ovulation by surgical ablation and replacement therapy, and the role of corpus luteum hormones in parturition (4) measure prolactin levels following mammary denervation and determine the effects of this manoeuver on milk composition (5) investigate the control of the changing milk composition by transfer of pouch young of various ages to suckled mammary glands. Milk will be analysed using standard electrophoretic and protein quantitation techniques.